Scientific investigation of the corrosion performance of magnesium and magnesium oxide primers on Al alloy 2024-T351 in field exposures

Abstract

The corrosion protection attributes of a magnesium oxide-rich primer (MgORP) on an AA2024-T351 substrate were evaluated after 2.5 y of coastal marine field exposure using several diagnostic electrochemical methods and nondestructive characterization techniques. The extent of protection afforded by Mg-converted and MgO pigments to exposed AA2024-T351 in the underlying substrate and within scribed coating defects was investigated. X-ray diffraction results showed that elemental Mg pigment was not entirely depleted after 4.25 y of exposure and that Mg(OH)2 was a prevalent corrosion product for magnesium-rich primer (MgRP), whereas there was limited indication of MgO pigment present in the MgORP after only 2.5 y of exposure and no evidence of crystalline corrosion product formation within the primer. Mg2+ deposited in scribes where bare AA2024-T351 was exposed in both the MgRP and the MgORP. The amount of Mg2+ that repartitions from the primer onto the surface of an artificial scribe increased throughout field exposure testing for MgRP. Thus, Mg(OH)2 precipitates at defects and scratches, providing a mechanism of chemical corrosion inhibition. The mechanisms associated with this process are assessed. The performance of MgORP coating was compared/contrasted with field-exposed (MgRP) to understand how a field-exposed and spent MgRP may continue to provide protection to the substrate after Mg depletion. Observations support the viability of MgRP for long-term exposure corrosion protection of susceptible aluminum-based assets based on this chemical protection mechanism.